If the ball is kicked from the in-goal area, where does a player have to stand to be onside? Do they have to be behind the kicker (as they would if the ball were kicked from the main field of play) or do they just have to be in their in-goal area?
The in-goal is considered part of the playing area (see Law 1 The ground).
A player can be offside anywhere in the playing area (see Law 10 Offside and onside in open play, verbatim paragraph 2).
The in-goal is treated no differently, and they are offside until they are put onside again by any of the methods outlined in the rest of Law 10.